(a) Field of the Invention
The present invention relates to a compensation device for convergence drift used in a cathode ray tube, and more particularly, to a device capable of reducing an occurrence of convergence drift due to electric charges accumulated on an inner surface of a neck portion thereof.
(b) Description of the Related Art
A cathode ray tube (CRT) is a device for displaying images on a screen by emitting electron beams from an electron gun assembly and landing the electron beams onto a phosphor screen. The electron gun assembly is mounted in a neck portion, and includes three cathodes for emitting electrons and a plurality of grid electrodes for generating electric lenses therebetween to control the focusing and accelerating degree of the electron beams.
After three electron beams are emitted, they are horizontally and vertically deflected by magnetic fields generated by a deflection yoke, and divided onto corresponding red, green and blue phosphor layers respectively through a shadow mask. Thus, the electron beams strike all pixels on the phosphor screen.
In such a CRT, a voltage supplied to a vacuum bulb when the CRT starts operation leaks to the neck portion with time because of a high potential difference between the grid electrodes and the neck portion. Consequently, electric charges in the bulb are continuously accumulated on an inner surface of the neck portion.
Because the accumulated charges generate new electric fields, they change the path of electron beams passing on the grid electrodes and vary the convergence degree of the beams. Thus, the charges cause "convergence drift" that allows deterioration of image quality.
To reduce the convergence drift, U.S. Pat. No. 4,868,454, 4,564,786 and 4,503,357 provide the use of conductive tape attached on an outer surface of the neck portion for early stabilization of the convergence drift.
However, the conductive tape that is simply attached on the neck portion is under a floating condition that makes a uniform potential state unattainable. Thus, the conductive tape is sensitive to outer influences, for example approach of charged materials and variation of humidity, so that it is difficult to operate in a stable fashion.
In particular, when the conductive tape is placed opposite to intervals between the grid electrodes, minute potential variation of the conductive tape has a great deal of influence on the electric lenses generated in the intervals. Such variation of the electric lenses impedes accurate control of the electron beams.
Also, when the conductive tape is grounded, it easily increases potential differences and intensity of electric fields between the grid electrodes and the neck portion. Thus, the grounded conductive tape has a tendency to increase the convergence drift as the electric fields increase the accumulated charges on the neck portion.